Components that need to be cooled are, for instance, compact electronic subassemblies having highly-integrated circuits (ASICs), and digital microcontrollers in which high power-loss densities could occur. When the electrical power consumption is high and, in particular, when transient load states occur, highly integrated electronic circuits may heat up very quickly due to the unavoidable Ohmic and dielectric losses. However, excessively high internal temperatures at the pn-junctions of the semiconductors can lead to malfunctions, premature ageing and breakdown of the entire subassembly, which is why the internal thermal energy must be dissipated to the system environment.
The heat dissipation takes place by passive diffusion cooling, for example, or by the technically complex convective coolant recirculation with the aid of turbo or positive-displacement pumps. The recirculated coolant fluid absorbs heat at the hot surface of a component and transmits it to a heat transfer element having a large surface. Normally, the surface of the heat transfer element has thin lamellae or ribs on the system surface. A media flow along the external surface has an advantageous effect that counteracts the boundary layer effects hindering the heat transfer.
A heat transfer takes place at a surface having a higher temperature to a surface having a lower temperature. When a thermal equilibrium is present, no energy exchange with the system environment occurs.
In addition to diffusion cooling and convective cooling, electrical cooling via Peltier elements is known also available. However, these elements are relatively expensive, increase the electrical power consumption of the electronics, and possibly use more space themselves than the electronic components to be cooled. For this reason, Peltier elements are not an option for wide-spread use.
In addition to convective coolant recirculation with the aid of turbo or positive-displacement pumps, passive coolant recirculation according to the principle of self-maintaining convection in the form of so-called thermo-siphons is also used. However, this presupposes an orientation of the heat flow parallel to the gravitational field. In computer electronics, heat pipes, which likewise operate according to the principle of passive coolant recirculation, are also used for cooling. A heat pipe is a hermetically sealed system for the transfer of heat by evaporative cooling and self-maintaining coolant convection. The boiling-point temperature of a coolant used in a heat pipe lies in the thermal operating range of a heat source to be cooled. The liquid phase wets the inner wall of a thin pipe, and the gaseous phase, driven by the gas pressure in the interior of the pipe, is able to flow to the heat sink and dissipate the evaporation heat to a heat exchange element there. The vapor condenses in the process. The fluid wets the internal walls of the pipe and flows back to the heat source propelled by the capillary effect. To enhance the flow based on the capillary effect, the inner wall of the heat pipe is usually provided with a capillary structure.
However, when thermal contact to the system environment is technically not feasible or not desired, latent heat stores, which absorb the heat from a heat source, may be used. This is particularly advantageous if transient thermal power peaks in the load profile of the application must be intercepted. For example, in a temporary rapid increase in the ambient temperatures, e.g., while driving slowly or while the vehicle is standing following a longer operating period in the full-load state of the engine, cooling by diffusion or air convection of electronic control devices in an engine compartment of a motor vehicle may possibly no longer be sufficient. In this case the advantages of latent heat storage come to bear. Instead of transmitting the lost heat to the system surface, for instance by diffusion or coolant convection, the heat is dissipated to a phase-change material PCM. In the coexistence range of the solid and fluid phases, as well as the in liquid and gaseous phases of a PCM, heat input does not lead to an increase in temperature, but to a change in entropy in the change of the state of aggregation or in the degree of order, for example, by a reduction of the magnetic polarization or the loss of the crystalline long-range order.
In a phase change due to the absorption of heat or the dissipation of heat, the temperature of the PCM remains generally constant. However, in the boundary layer to the heat source, the local temperature of the liquid phase, for example, may become higher than the melting temperature as the layer thickness increases, due to its poorer thermal conductivity in comparison with the solid phase. A temperature gradient forms between the solid phase and the component to be cooled.
Latent heat stores containing a phase-change material which are employed for cooling are described in PCT Application No. WO-A 2007/075130 and U.S. Published Patent Application No. 2002/0164277, for example.